1. Field of the Invention
The present invention relates to a an electric motor which is required to output a large amount of power at sporadic intervals such as a starter motor for use in automotive engines or similar types of application, and more specifically to an improved arrangement which dissipates the heat produced during operation and which permits prolonged cranking without thermal damage.
2. Description of the Prior Art
FIG. 1 shows a prior art automotive starter motor arrangement. In this arrangement a DC electric motor 1 includes a stator 2, a rotor 3, a commutator 4 and a brush 5. As shown, the stator 2 is disposed within a steel housing 6 and about the rotor 3. A brush holder 8 secures the brush 5 in the appropriate position.
The DC-motor 1 is operatively connectable with the crankshaft of the engine (not shown) through a planetary type reduction gear 11 and a pinion 10 mounted on an output shaft 9.
An electromagnetic clutch 12 is arranged to selectively engage the shaft extending between an over-running clutch 13 and the reduction gear 11, by way of a shift lever 12a, in a manner so as to move the pinion 10 into mesh with a non-illustrated ring gear and therefore establish a drive connection between the pinon 10 and the motor 1.
During cranking of the engine, a large amount of power must be produced by the motor 1. This production is accompanied by the generation of a substantial amount of heat which tends to be transferred to the pole pieces 7 disposed on the inner periphery of the steel housing 6.
As the size of the starter motor has been reduced gradually over a period of time in accordance with the demand for more compact engine arrangements, the current models are such that the pole pieces 7 are usually made of alloys consisting of Nd, Fe, B or Al, Ni combinations which exhibit a loss of magnetism at elevated temperatures.
Accordingly, a drawback has been encountered in that these types of magnets tend to become excessively heated, e.g., from 320.degree. to 330.degree. C., and thus approach their Curie points in the event that engine cranking is continued for any length of time. As a result, it is necessary to appreciably reduce the length of time for which engine cranking can be continuously carried out in order to avoid an overheating situation wherein the starter motor becomes "burned out".